There have always been restrictions on the number of aircraft that can be landed before 0600 local at Heathrow. When I've been on a flight expecting serious tailwinds, we've often delayed pushback or sat in the penalty box so that we arrive on schedule instead of early.
I think BST comes to effect the previous weekend, and as Hong Kong doesn't operate any daylight savings and stays the same all year round, this would result in the flight arriving an hour early throughout BST.
I'm guessing this could be the reason behind the change in departure time.
Cheers,
Mike